Welcome from North Wales!
I agree that you do sound like you have a square head on your shoulders, but here's just a few things for you to consider. I don't know if you rent or own your flat but some flat buildings and landlords will have breed restrictions, so you will have to make sure you have permission to own such a big dog there.
Also do you plan on going to college or uni in the near future? Seeing as you're in your early teens, you have another 4-5 years left before this would be an issue, but would it be fair to jet off to Uni, leaving the 4/5 year old dog that you played the majority part in raising back with your parents?
Have you met any Dobies? Have your parents met any? Both puppy and adult? Were you thinking male or female? Would you consider adopting a dog from rescue? There are several beautiful candidates that may suit your family situation awaiting homes right now! Dobermanns Awaiting Rehoming - Dogs Needing Homes Now | Dobermann Welfare Association doberman dobermann rescue Dobermann Rescue Dobes in Rescue
I will PM you with another user's info. They have a beautiful dog out of a pretty good breeder in Scotland who has been known to do the required health testing, including Holter/Echo monitoring for Dilated Cardiomyopathy. Something that is very important to have done on the parents of puppies, I wouldn't be rushing to give my money towards a puppy from a breeder that didn't test their dogs for cardio annually. Maybe they would be willing to meet you and let you meet their dogs or even get you in touch with their breeder to meet their dogs.
Most UK breeders will only test hips, eyes (PHPv), and vWD status of their dogs. They don't tend to bother listing details such as Brucellosis or Thyroid, which are both important tests that should be done prior to breeding. Cardio is also something to be careful about. A vet check with a stethoscope is not enough, both parents should have an echo/holter prior to the breeding. Don't fall for the excuse that dogs under the age of 5 or 3 can't or don't need to be tested, you can read Brandy's story, she was only 4 months old: Aritaur Dobermanns - Breeders of Dobermanns for Work, Show and Pet
Aritaur's website is a brilliant resource for the UK dobe folk
I wish you the best of luck in finding your perfect companion!